The present invention relates to a golf club shaft in which the directivity of a golf ball that has been hit by the golf club can be enhanced.
In order to increase the rigidity of the base end side of a golf club shaft at which a grip is attached, the golf club shaft is formed into a tapered shape in which the diameter of the golf club shaft is gradually increased at it comes from the front end to the base end.
Usually, a kick point is set at a position distant from the front end of the shaft by 30 to 50% with respect to the overall length of the shaft. When the kick point is set at a position in the above range, a golf ball is not raised too high or too low after it has been hit by the golf club, so that the golf ball can fly along a desired trajectory.
In this connection, the golf club is constructed in such a manner that a heavy head is attached to the front end of a shaft, and further the center of gravity of the head is made to deviate toward the back side of the head with respect to the axial direction of the shaft. As a result of the above construction, when a golfer swings down the golf club, the head is twisted in the circumferential direction of the shaft at a position where the torsional rigidity of the shaft is lowest. A direction of torsion of the head is different from a direction of deflection of the shaft at the kick point. Accordingly, at the moment of the impact of the head against a golf ball, an advancing direction of the head does not coincide with a direction of the face of the head. Accordingly, even if the golf ball is hit at a sweet spot on the face of the head, it is impossible for the golf ball to be shot in the target direction.
As shown by a broken line in the graph of FIG. 2, in the conventional shaft of this type, the torsional rigidity of a small diameter portion of the front end of the shaft, that is, the torsional rigidity of the end portion of the shaft at which the head is fixed is lowest. However, the present applicant has discovered that the directivity of a golf ball, which has been hit by a golf club, can be enhanced when a position of the shaft at which the torsional rigidity is lowest is located at a position close to the kick point or nearer to the base end side of the golf club shaft than the kick point.
In this connection, Japanese Unexamined Utility Model Publication No. 55-1311275 discloses a shaft 9 illustrated in FIG. 9, the detail of which will be described as follows. A sheet 1 impregnated with synthetic resin, on which carbon fibers are arranged in one direction, is wound and formed into a shaft body 3. On the front end side and the base end side of the shaft body 3, kick point adjusting sheets 5, 7 formed from sheets, on which carbon fibers are arranged in the direction of angle 0 .right brkt-bot. with respect to the axis of the shaft body 3, are respectively wound round the shaft body 3, and an interval of the two kick point adjusting sheets 5, 7 are arbitrarily adjusted, so that the kick point position can be adjusted.
When an amount of high strength fibers such as carbon fibers arranged in the direction of angle 0 .right brkt-bot. with respect to the axis of the shaft 9 is changed, it is possible to adjust the position of the kick point. However, even if the amount of high strength fibers arranged in the direction of angle 0 .right brkt-bot. with respect to the axis of the shaft 9 is changed, it is impossible to adjust a position where the torsional rigidity is low.
That is, in order to adjust the torsional rigidity of a shaft, it is necessary to change an amount of high strength fibers which are arranged on the circumferential direction of the shaft.
When a small diameter portion 11 is formed on the shaft 9 as illustrated in FIG. 9, the golf club looks unattractive, and further there is a possibility that the shaft 9 is damaged at the small diameter portion 11 in the case of stress concentration.